The statements in this section merely provide background information related to the present disclosure and do not constitute prior art.
H.264/AVC uses Instantaneous Decoding Refresh (IDR) picture for enabling an encoded bitstream to be randomly decoded from a specific time position. As illustrated in FIG. 1, an encoder deletes decoded pictures from a Decoded Picture Buffer (DPB) and clears the DPB immediately before encoding IDR picture. As in the case of the encoder, a decoder also clears a DPB immediately before decoding IDR picture.
Pictures to be encoded or decoded after the IDR picture of FIG. 1 are encoded with reference to the IDR picture and pictures encoded and decoded after the IDR picture.
An IDR picture encoding method is substantially identical to an I picture encoding method. However, the IDR picture encoding method supports a random access function that enables a decoding process to be performed from a random IDR picture time point by inserting IDR picture.
When encoding inter-pictures, H.264/AVC predicts current picture by using pictures previously encoded and decoded. As illustrated in FIG. 2, in the case of P picture, a motion estimation is performed by using a past picture existing prior to current picture on the basis of display order. A residue signal between a current block and a block determined through the motion estimation is encoded.
As illustrated in FIG. 3, in the case of B picture, a motion estimation is performed by using past picture existing prior to current picture and future picture existing after the current picture on the basis of display order. Then, a residue signal between a current block and a block determined through the motion estimation is encoded.
According to the H.264/AVC standard, one or more sheets of reference pictures can be used for inter-picture estimation with respect to each prediction direction. Due to this function, H.264/AVC exhibits superior compression performance to other picture compression standards.
H.264/AVC uses IDR picture for supporting a random access function. As illustrated in FIG. 4, an encoder and a decoder clears a DPB immediately before encoding and decoding IDR picture. Hence, the inventor(s) has noted that with respect to some of B pictures displayed before the IDR picture, bidirectional motion estimation cannot be performed during encoding, and therefore, only unidirectional motion estimation is performed. The inventor(s) has experienced that consequently, the encoding efficiency of pictures performing only the unidirectional motion estimation is reduced.
In addition, as illustrated in FIG. 5, specific P pictures displayed after the IDR picture can use only one IDR picture for the reference picture, but cannot use a plurality of pictures prior to the IDR picture for the reference picture. Therefore, the inventor(s) has experienced that the encoding efficiency of the P picture may be reduced.